French Patent No. 77.31719 filed on Oct. 21, 1977 in the name of the present Applicant disclosed these new drugs as having a very high therapeutic index due to the very low toxicity of 5-methoxypsoralene compared to that of other psoralenes. The patent made clear that one could use therapeutic doses of 5-methoxypsoralene up to five times greater than those used for the 8-methoxypsoralene and advocated the administration of oral doses of 20 mg to 300 mg of 5-methoxypsoralen per day, followed by a daily exposure to A ultraviolets, at a rate of 1.5 to 10 joules/cm.sup.2.
For topical administration, the patent advocated 5-methoxypsoralene concentrations between 100 ppm and 1000 ppm. The experimentations of the therapeutical activity of the new drugs had been carried out in the treatment of psoriasis, vitiligo, atypical eczema and mycosis fungoides.
It should be recalled here that the mycosis fungoides is a well known skin cancer of the lymphoma type, that is characterized by the proliferation of the lymphocyte cells more present in the lymph, that is the interstitial liquid irrigating notably the dermis and epidermis.
The clinical experimentations which resulted in Patent No 77 31719 had been of course applied both on patients affected with psoriasis and patients affected with skin cancer, since one and the other of said illnesses are characterized by a more rapid than normal proliferation phenomenon of the cutaneous cells. Such a phenomenon of a too rapid proliferation of the cells is at the level of the deoxyribonucleic acid molecule present in the cell nucleus.
But it is known that the 5-methoxypsoralene, under the energetic action of a UV.A radiation gets fixed on each of the two helixes of the DNA molecule and therefore blocks any transmission of the message along said helixes, thereby stopping altogether the proteine synthesis and the cellular multiplication. In the normal cells, said blocking of the DNA helixes is cought up in speed by the DNA self-repair phenomena either by excision or by replication.
But in the case of cells affected with psoriasis or cancer, that is cells where the multiplication phenomena are very accelerated, the self-repair systems do not have time to set to work. In the same cells however the psoralene, by forming bridges between the two DNA helixes, stops the anarchical cellular multiplication. This explains that the 5-methoxypsoralene does not disturb in any way the multiplication of the normal cells and blocks selectively the diseased cells with accelerated proliferation which are the psoriatic cells and the cancerous cells.